In the field of microprocessor chip packages, an effective transfer of thermal energy from a microprocessor chip to a heat sink is important for thermal management of the microprocessor chip.
A known type of active cooling device is an impingement cooler in which coolant flows through channels in the impingement cooler and is then fed through a jet and impinges on the back side of the microprocessor chip. An external pump is used to circulate the coolant between the back side of the microprocessor chip, where heat is absorbed, and a radiator. Heat is subsequently dissipated from the radiator to ambient using, for example, a fan and an air heat exchanger.
A further type of known cooling device for cooling microprocessor chips is a microchannel cooler, as described in “A Practical Implementation of Silicon Microchannel Coolers for High Power Chips” by E. G. Colgan et al., 21st IEEE SEMI-THERM Symposium, 1-7 (2005). A manifold part and a channel part are bonded together to form multiple heat exchanger zones and cooler fins. A coolant flows through parallel channels in the device and the flow length between a coolant inlet and a coolant outlet is approximately 3 mm.
A third type of known cooling device for laminar convective heat transfer from a solid to a liquid is tree-like microchannel net. This structure efficiently removes heat like a classical microchannel cooler but does not require an additional manifold (Senn and Poulikakos, J. Power Sources, 130, 178-191 (2004).
Generally, the transfer of thermal energy is proportional to the area of a heat exchange surface of the microprocessor chip and also increases in near proportion with the velocity of a coolant moving along the heat exchange surface. A further means of increasing the amount of heat exchanged is to employ geometries which impinge the coolant flow against the exchange surface. This has the effect of decreasing the thickness of the thermal boundary layer which exists at the surface, thereby enhancing transport of heat from the hot solid to the bulk of the coolant flow.
An impingement cooler having a single jet may provide heat transfer for a localized area covered by the impinging jet flow field. Heat transfer performance may be relatively improved by reduction of the single jet diameter. However, this introduces the problem of the impinging jet flow field being reduced. To overcome this problem it is known to provide a jet impingement array.
A jet impingement array generally comprises a two-dimensional grid pattern of multiple impinging jets set in a manifold. A coolant fluid ejected from each impinging jet in the array, impinges the back side of the microprocessor chip, and then exits the space between the jet impingement array and the microprocessor chip, known as the impingement gap, through one or an array of outlets. Problematically, due to deflection of peripheral impinging jets, the drainage flow may accumulate at the edge of the impingement gap, which may reduce heat transfer performance. Further, collision of jet flow from neighboring impinging jets can result in mutual cancellation of flow speed. These localized regions of low or no flow are known as stagnation points. Also, where jet flow directly below an impinging jet does not flow away, a stagnation point occurs.
Such an impinging jet generally forms an inhomogeneous flow field. From the stagnation point at the centre, the flow field speed increases as radial distance from the stagnation point increases, until a maximum speed is reached at a radial distance of approximately 2-3 times the diameter of the impinging jet. As the radial distance from the stagnation point increases further, the flow field speed decreases with the second power of the radial distance from the impinging jet.
A coefficient known as the convective heat transfer coefficient, h, is used for characterizing convective heat transfer rates into the fluid phase. This coefficient is measured in units of W/cm2-K, i.e. Watts per centimeter-squared Kelvin. In a situation where a fluid interfaces with a solid body, a thermal boundary layer of fluid may form. Then, h is equal to the local heat flux at the solid-fluid interface divided by the difference in temperature between the fluid bulk, or temperature of the fluid beyond the thermal boundary layer, and the surface temperature of the solid. In impingement jets or microchannels, both the value of h and of the bulk fluid temperature depend strongly on location, so that for a meaningful comparison of thermal performance between geometries one must define an effective heat transfer coefficient heff based on averaged quantities.
      h    eff    =            q      ″                      T        fsi            -              T        inlet            where q″ is a heat flux area-averaged over the fluid-solid interface, Tfsi is an effective temperature at the fluid-solid interface which must be derived on the basis of one or several temperatures measured at locations on the solid being cooled (e.g. on the junction side of a microprocessor chip), and Tinlet is the mass flow averaged temperature of the coolant when it enters the heat exchanger.
An average convective heat transfer coefficient of jet impingement cooling, where the fluid is water and an array of impingement jets have round nozzles, has been estimated using Martin's correlation (Martin, H., “Heat and Mass Transfer Between Impinging Gas Jets and Solid Surfaces”, Advances in Heat Transfer, Academic Press, vol. 13, pp 1-60, 1977). Martin's correlation provides a relationship between an expected thermal performance of an array of impingement jets having round nozzles as a function of all key geometric parameters and fluid flow conditions, in a standard dimensionless form, and has been tested against a large set of experimental data. Martin's correlation provides information allowing an optimization of the geometric parameters of an impingement cooler for a given jet nozzle diameter.
It can be calculated, using Martin's correlation, that the cooling performance of a jet impingement cooler increases with decreasing jet pitch and jet nozzle size. Scaling a jet impingement array to smaller dimensions enlarges the proportion of a pressure drop from the manifold as impinging jet nozzle diameter is reduced, because the jet pitch is also reduced. A practical minimum jet nozzle diameter can be approximately 80 microns, μm, where the impingement gap G would be approximately 320 μm and the jet pitch P approximately 300 μm, which would correspond to values of heff between 10 and 15 W/cm2-K, depending on the jet flow velocity. Thus, as jet flow velocity increases, the required fluid flow and pressure drop also increase.
Jet impingement arrays as discussed so far are problematic in that they generate a flow field which is inhomogeneous not only locally on the scale of the distance between individual jets but also globally over the area of the larger surface to be cooled, because jets positioned radially outwards from the array center are deflected by the radial flow of coolant coming from the more centrally positioned jets. Thus, the rate of cooling may be substantially less at the periphery of the chip than at the center. Similarly, microchannel coolers generally present poor long-range cooling uniformity because the fluid warms up along the relatively long channels.
Drainage of a jet array in a closed system is done by one exit port—a concept which scales poorly due to the accumulating crossflow. In Saad et al., 1992 the adverse effect of crossflow in a slot nozzle array is alleviated by the use of distributed return by exit ports interleaved with the inlet port array. This reduces the impingement configuration to a collection of individual cells which form a self-contained unit and can be used in arbitrarily large arrays i.e. this system scales with the size of the surface to be cooled. One of the principal issues of this approach is to provide feed and return flow to the individual cells in a way that is compatible with one sided access to the inlet and outlet arrays and retain the property of low pressure drop.
From scientific literature it is known that from a thermal performance point of view it is desirable to scale down the physical dimensions of an impingement cell. Thus, in order to achieve a desirable thermal transfer performance it is preferable for jet nozzles to be relatively small, in the range of 20-100 μm.
It is a further problem associated with jet impingement coolers that heat transfer is only possible through the impinged surface. Surface enlargement done by microscopic features only slightly improves the overall performance of the jet device since they create zones of recirculation and zero flow. For example, in Hansen and Webb, 1993, improvement factors of 1.5 to 4 are seen for surface enlargement.
A problem also associated with conventional jet impingement coolers that heat may generally only be removed through the contact between coolant fluid and hot surface and that no heat may be conducted to the upper part of the cooler where also a jet plate or manifold structures could contribute to the heat exchange. Furthermore, when utilizing parallel manifold structures in conventional jet impingement coolers there is a notable increase in pressure drop in the manifold as nozzle pitch decreases. Thus, conventional manifold structures do not function adequately at the required reduced physical scales.
It is an aim of the present invention to provide an arrayed cooling device which mitigates the problems of the known art.